The invention describes improvements in the commercial-scale processing of nickel and cobalt containing laterite ores for the recovery of these metals, by reacting such ores with sulphuric acid at elevated temperatures and pressures
For over a century, nickel laterite ores high in magnesia, relatively low in iron, and enriched in nickel, commonly referred to as garnierite ores or saprolite ores, have been processed by pyrometallurgical means to produce either a ferronickel, a Class II nickel product that could go directly to market for the production of stainless steels, or to produce an intermediate sulphide xe2x80x9cmattexe2x80x9d product that could go to refineries for conversion to either Class I or Class II nickel products. A good portion of the cobalt would be lost, some in the slag during the smelting stage, and in the case of ferronickel most of the cobalt would be present as a product impurity of no value. Such pyrometallurgical processes involve drying the humid ores, preheating them with or without effecting a partial reduction, and subsequent reduction smelting at high temperatures in electric furnaces. It is axiomatic that such pyrometallurgical processes consume high amounts of energy per unit of nickel production, and in most cases result in complete loss of value of the cobalt that accompanied the nickel in the ore.
About half a century ago, an ammoniacal leaching process was developed and commercialized which could treat laterite ore relatively high in iron and of lower nickel content than the garnierites and saprolites. It employed a combination of pyrometallurgical and hydrometallurgical technologies. The laterite ore is first dried and then subjected to partial reduction in Herreschoff furnaces or rotary kilns, at elevated temperatures but well below smelting temperatures, to selectively reduce the nickel and cobalt but only partially reduce the iron. This partially reduced calcine is then quenched and leached in ammoniacal carbonate solutions to dissolve nickel and cobalt; and the nickel is subsequently recovered from the ammoniacal leach solution as a nickel hydroxide/carbonate which would then be converted to a Class II nickel oxide or to utility-grade nickel. In some cases the nickel solutions would proceed to electrolytic refining for the production of refined nickel. Nickel extractions seldom exceed 80% and cobalt extractions seldom exceed 45%. While this hybrid pyrometallurgical-hydrometallurgical process could treat the high-iron, low-magnesia and low-nickel laterite ores, often referred to as limonite ores, and is less demanding of energy than the smelting process, in actual continuous practice, the nickel recoveries often fall below 75% and cobalt recoveries below 40%.
Research in the early 1950""s demonstrated that by subjecting the high-iron, low-magnesium and low-nickel laterite ores, that is the limonites, also containing significant quantities of cobalt, directly in their humid state to sulphuric acid at elevated temperatures and pressures, that nickel and cobalt extractions of over 90% could be achieved with the energy requirement only a fraction of that required by the smelting or ammoniacal leaching processes. While this technology heralded a new era for the production of nickel and cobalt, only one commercial plant was built at Moa Bay in Cuba. This plant confined itself to the processing of limonites very low in magnesia content, i.e., with less than 1% magnesium oxide, and operated at around 240xc2x0 C. and 475 psig. The plant which is in operation today, employs pachuca-type autoclaves which rely on the process steam to provide both the heat requirement and the agitation which is inadequate and promotes build-up inside these autoclaves which in turn necessitates frequent shutdowns for cleanouts. The product of the Moa Bay plant is an intermediate nickel-cobalt sulphide, which is sent overseas for refining to marketable nickel and cobalt end products.
The value of this new hydrometallurgical technology that could treat humid ores directly without drying and which yields impressively high extractions of nickel and cobalt, became more and more appreciated as a result of the energy crises of the 1970""s and 1980""s and as a need grew for new sources for cobalt outside of Zaire and Zambia whose production had dropped off drastically. At the same time, the development and demonstrated success of large-scale mechanically-agitated compartmentalized autoclaves in other industries such as the gold industry, gave added interest for application of such reactors to the processing of nickel-cobalt laterites. In such reactors, the requirement for process steam and for agitation are managed and adjusted independently one of the other. Furthermore, extensive research developmental work carried out by P. C. Duyvesteyn, G. R, Wicker, R. E. Doane of Amax Extractive and Development Inc. xe2x80x9cAn Omnivorous Process for Laterite Depositsxe2x80x9d, International Laterite Symposium, Evans, Shoemaker, Veltman Eds., TMS-AIME, Kingsport Press, Kingsport Tenn., 1979, demonstrated that enhanced results could be realized at somewhat higher temperatures of around 270xc2x0 C. and corresponding pressures of around 800 psia; and that this new technology employing mechanically-agitated reactors need not limit itself to the very low-magnesia laterite ores, but could be applied to ores containing several percent of magnesia. Of course, acid requirements increase significantly as the magnesia increases as does the requirements for neutralizing agents. The greatest impetus to proceed with this new technology comes from engineering and economic analyses which indicate that hydrometallurgical process plants could be constructed at a capital cost per unit of annual nickel and cobalt production substantially below that of the established conventional processes and would yield a unit cost of production which permits economic treatment of limonites with as little as 1% of nickel, material that up until now had been considered as overburden and uneconomical to process, i.e., material that previously could not be classified as ore. This has led to the construction of three separate acid pressure leaching plants in Australia, with commissioning in 1999/2000.
U.S. Pat. No. 4,541,994, 1985, assigned to Lowenhaupt et al. speaks of reacting xe2x80x9ccoarse, magnesium rich fractionsxe2x80x9d with partially neutralized pregnant liquors produced by high pressure leaching, at lower pressures, and claims carrying out of such reactions xe2x80x9cat a pressure of from atmospheric to about 300 psigxe2x80x9d, also xe2x80x9cwherein said pressure is atmospheric and said temperature is below 80xc2x0 C.xe2x80x9d, also xe2x80x9cwherein said temperature is about 60xc2x0 C.xe2x80x9d, and also xe2x80x9cwherein said temperature is ambientxe2x80x9d. Their atmospheric leach tests Nos. 7, 8, 9 and 10 at 80xc2x0 C., for example, demonstrated that nickel and cobalt tend to be upgraded in the fine fractions and magnesium in the coarse fractions. In these tests, the Mg:Ni ratio in the +200 mesh size in relation to the Mg:Ni ratio in the xe2x88x92200 mesh averaged 2:1; and the Mg:Co ratio in the +200 mesh size in relation to the Mg:Co ratio in the xe2x88x92200 mesh size averaged 2.1:1. Only the xe2x88x92200 mesh size would proceed to acid pressure leaching. While less acid would thus be required per unit of nickel and cobalt to yield high extractions in the pressure leach, overall nickel and cobalt recoveries would be greatly decreased.
Currently, in preparation for the pressure leaching, the humid predominantly limonitic laterite ores are pulped with substantial quantities of calcium-free water either from a xe2x80x9cfreshxe2x80x9d water source or with de-ionized saline water, to a pulp density usually under about 40% solids; and excess acid is added to the autoclaves to effect the desired leaching in 60 minutes or less, when employing reaction temperatures of up to 270xc2x0 C.
It is well understood and appreciated by those familiar with acid pressure leaching of laterite ores, that the pH of acidic leaching solutions is different at elevated temperatures than at temperatures below 100xc2x0 C.; and that the solubility of metals such as nickel, cobalt, manganese, and magnesium drops off drastically at temperatures above about 150xc2x0 C. Accordingly, sulphuric acid well in excess of that theoretically required to sulphate the desired metals must be employed to maintain an adequate level of acidity at the elevated reaction temperatures, as well as to enhance the kinetics of the sulphating reactions. The net result is that the leachate emanating from the autoclaves after being cooled and de-pressurized, can contain as much as 30 grams per liter to as much as 50 grams per liter of free sulphuric acid.
Typically, with low-magnesia limonite ores the sulphuric acid addition to the feed is about 30% by weight of the ore (on a dry weight basis); and the free acid in the leachate could represent at least 25% and as much as 40% of the initial acid addition under certain operating conditions. Before proceeding to recovery of the nickel and cobalt from the leachate by any of the conventional means of precipitation with hydrogen sulphide or by more-recently developed solvent-extraction or ion-exchange technologies, or by precipitation by more common basic neutralization agents such as magnesia or sodium oxides or carbonates, it is usual to carry out a preliminary partial neutralization with limestone to a pH of 3.5 to 4.5 in order to neutralize the bulk, that is over 95%, of the free acid and to precipitate most of the ferric iron. At this stage the partially neutralized leachate would be virtually saturated with calcium sulphate. The overall impact of this partial neutralization technique is that a significant tonnage of excess acid is wasted, a significant tonnage of extra limestone is required to neutralize the excess acid, the partially-neutralized pregnant solution is saturated with calcium sulphate, following metal recovery the barren solution cannot be recycled to preparation of new feed for the autoclaves, and substantial quantities of process effluent needs to be discharged to the external environment after final neutralization to lower the concentrations of base metal contaminants.
As already stated, one of the basic tonnage materials required to carry out acid pressure leaching, besides the ore and sulphuric acid, is water. It is necessary to pulp and dilute the ore feed to about 40% solids or lower. It is highly desirable, and in most cases essential, that this make-up water be free of calcium so as to avoid problems that could arise from calcium sulphate precipitation particularly in the preheating system at the feed end of the pressure system autoclaves. Thus, commercial installations rely on fresh water sources if such are available, or arrange for de-ionization of saline waters. The tonnage of calcium-free water required is very large, usually in the order of a tonne of water for every tonne of raw humid laterite ore. While adequate quantities of fresh water may be available for initial demonstration plants, it is unlikely that there would be enough available for any large-scale operations and expansions. Furthermore, there is a serious environmental consideration in that every tonne of fresh water taken into the process usually results in a comparable tonnage of process water that must be discarded eventually to the sea, and which could carry certain quantities, albeit minute quantities, of base metals and other contaminating elements.
It is an object of the present invention to provide a process of hydrometallurgical treatment of laterite ores of the limonitic type for the recovery of nickel and cobalt which reduces the amount of fresh water needed for pulping the feedstock and the amount of sulphuric acid used in the chemical leaching step.
In the improved method of the present invention, a significant portion of the xe2x80x9cmother liquorxe2x80x9d emanating from the autoclaves is recycled to the feed preparation stage thereby substituting for all or at least a major proportion of the fresh water or de-ionized water that must be added and also supplying a portion of the amount of sulphuric acid that is required for leaching. Concomitantly with the major savings in fresh water or de-ionized water requirements, a significant reduction in new sulphuric acid requirements is effected along with a corresponding saving in limestone and lime required for subsequent neutralizations. A further benefit results from the fact that less process waters need to be expelled to the external environment Thus, several significant processing advantages and benefits are simultaneously realized.
The present invention provides a process of leaching a nickel and cobalt containing predominantly limonitic portion of a laterite ore profile, comprising the steps of:
a) preparing a feedstock of a predominantly limonitic portion of a laterite ore containing nickel and cobalt;
b) pulping said feedstock with a liquid to produce a pulped ore;
c) adding an effective amount of sulphuric acid to the pulped ore to produce a sulphuric acid solution, agitating and leaching said feedstock in said sulphuric acid solution at an elevated temperature under pressure for a selected period of time whereby metal oxides are leached from said ore to produce a leach pulp;
d) separating said leach pulp into a mother liquor solution and a first thickened leach pulp, wherein said liquid used to pulp said feedstock in step b) includes a selected amount of said mother liquor solution; and
e) recovering nickel and cobalt products from said first thickened leach pulp.
In a variation of the present invention, acid efficiency may be further increased by reacting the first thickened leach pulp with highly-serpentinized, high-magnesia nickel saprolite ore, at atmospheric pressure and preferably above 90xc2x0 C. and below 100xc2x0 C., to achieve partial neutralization of the excess acid before it passes on to further neutralization with limestone and subsequently passing on to a decantation step (preferably using counter-current decantation) for solids-liquid separation and recovery of a clarified pregnant solution containing the nickel and cobalt values originating from both the limonitic ores treated at high temperatures and pressures and the highly-serpentinized saprolitic ores treated subsequently by atmospheric leaching.
In a further variation of the present invention, additional advantages are realized by completely eliminating the requirement for limestone and lime by carrying out preliminary partial neutralizations firstly with a highly-serpentinized high-magnesia saprolite ore and subsequently by the use of magnesite, MgCO3 or magnesia, MgO before passing onto solid-liquid separation and recovery of the clarified pregnant solution. The saprolite ore contributes nickel and a lesser amount of cobalt units and reduces substantially the quantity of the other neutralizing agents that would otherwise be required. When producing an intermediate nickel-cobalt product, final neutralization could be effected by any non-calcium basic oxides such as magnesia, or sodium-based oxides, carbonates or hydroxides. The metal values could alternatively be precipitated with H2S or sodium sulphide compounds; or could be recovered by either solvent extraction means or with chelating resins.